Electron Beam Focusing in Gas and Plasma-Filled Transport Cells on Rits

Abstract

Summary form only given. In flash X-ray radiography, one of the key goals is to focus an electron beam onto a high-Z-target to generate X-rays. One of several goals is to create a small radiographic spot. The paraxial diode, which utilizes a gas-filled transport cell to focus the beam into a small spot, has been investigated for several decades. It has been shown in simulations using LSP, a particle-in-cell code, that the primary limitation to achieving a small spot size is due to time-dependent net currents in the transport cell which cause the beam's focal plane to shift axially during the timescale of the pulse. This movement, often referred to as ''beam sweep, smears out the radiographic spot, leading to a larger size than is desired. Recently, calculations with LSP, demonstrate that replacing the gas with a pre-ionized plasma with density on the order of 1016 cm-3 significantly slows down the net current evolution, and yields a more stable focal plane and thus decreased time-integrated spot size. Recent experiments utilizing gas and plasma-filled transport cells have been performed on the RITS accelerator (4.5 MV, 20-30 kA beam current 60 ns FWHM) to study these predictions. The most recent results and analysis from these experiments are presented